The present invention relates generally to a pulse code modulation modem system. More particularly, the present invention relates to an upstream transmission scheme for transmitting data from a client modem coupled to a digital telephone network via an analog connection to a central site modem coupled to the digital telephone network via a digital connection.
FIG. 1 is a block diagram representation of a prior art V.90 modem system 100. V.90 modem systems (i.e., systems compliant with the ITU-T Recommendation V.90) utilize pulse code modulation (PCM) techniques in the downstream direction and conventional V.34 voice band techniques in the upstream direction. As used herein, the downstream direction refers to transmissions from a digital PCM modem (DPCM) 102 to an analog PCM modem (APCM) 104, while the upstream direction refers to transmissions from APCM 104 to DPCM 102. In the context of a common practical application, APCM 104 may be considered to be a client modem and DPCM 102 may be considered to be a server modem or a central site modem.
APCM 104 is coupled to a digital communication network, e.g., the public switched telephone network (PSTN) 106 via an analog connection 108, e.g., the local loop associated with a user""s telephone line. Analog connection 108 may be connected to a central office 110, which in turn is coupled to PSTN 106 via a digital connection 112. DPCM 102 is coupled to PSTN 106 via a digital connection 114. Modem system 100 takes advantage of the digital nature of PSTN 106 to enable downstream transmission at a theoretical data rate of 56 kbps.
Upstream transmissions in modem system 100 are limited by the quantization noise introduced by the codec resident at central office 110. The central office codec filters incoming signals before an analog-to-digital conversion. The filters at the codec remove low frequency components below approximately 100 Hz (the xe2x88x923 dB cutoff frequency) and high frequency components above approximately 3.5 kHz. Without taking advantage of the details of the central office codec, the upstream channel can be modeled as a channel having a signal-to-noise ratio (SNR) of about 37 dB. Thus, even if an upstream signal is adequately equalized by DPCM 102, the overall performance will be limited by the 37 dB SNR. Consequently, V.34 modems (and upstream V.90 transmissions) are currently limited to a maximum theoretical data rate of 33.6 kbps.
In a V.34 system, a form of Tomlinson-Harashima precoding is employed to pre-equalize the transmit signal in a manner that compensates for the channel characteristics. The transmit modem includes an equalizer structure that is tuned after a training procedure during which the receive modem trains an equivalent equalizer structure in response to a known training sequence. The receive modem communicates the equalizer filter tap settings back to the transmit modem for use in the precoding scheme. However, merely shifting the equalization function to the transmit modem may result in a dramatic increase in transmit power. Accordingly, precoding is applied such that the transmit power penalty is decreased. Tomlinson-Harashima (and other) precoding techniques are generally known to those skilled in the art. For example, such precoding techniques are set forth in detail in Lee and Messerschmitt, DIGITAL COMMUNICATION (Kluwer Academic Publishers, 2d ed., 1994), the content of which is hereby incorporated by reference.
PCM modulation for the upstream channel has been proposed in an effort to increase the upstream data rate in V.90 modem systems. In other words, the PCM scheme utilized for the downstream channel (or a modified version thereof) may be utilized for upstream data communication. Using upstream PCM modulation, theoretical data rates up to 48 kbps can be achieved if suitable equalization and processing are performed by modem system 100.
The Tomlinson-Harashima and V.34 preceding techniques are not effective in the context of a data communication system that employs a PCM modulation scheme. These prior art precoding techniques are based on the fundamental assumption that the transmit signal points are spaced apart in a linear or uniform manner. In other words, any two xe2x80x9cneighboringxe2x80x9d signal points in a V.34 modem system are separated by the same distance. In contrast, mu-law and A-law signal point constellations (either of which may be used in a practical PCM modem system) and constellations derived from mu-law or A-law constellations include signal points that are nonlinearly spaced. For example, signal points associated with relatively lower transmit power are spaced closer together than signal points associated with relatively higher transmit power.
Due to the inapplicability of V.34 preceding for PCM signal point sequences, a need exists for an appropriate technique that is compatible with upstream PCM transmissions.
A data communication system in accordance with the present invention provides an improved technique for providing PCM transmissions in the upstream direction from a client device to a server or central site device. The techniques of the present invention may be applied in the context of a modified V.90 modem system that utilizes PCM encoding in the upstream direction. The client modem device utilizes pre-equalization to compensate for characteristics of the upstream channel and a preceding technique to achieve upstream data rates that can exceed the maximum theoretical upstream data rate associated with conventional V.90 modem systems. The precoding technique does not rely on known Tomlinson-Harashima or V.34 precoding schemes.
The above and other aspects of the present invention may be carried out in one form by a modem device for transmitting data to a remote device over an upstream channel. One such modern device includes a data mode mapper configured to map digital data into a plurality of equivalent signal point sequences, where each signal point is selected from a nonlinearly-spaced signal point constellation. The modem device also includes a cost metric analyzer configured to obtain a measurable quantity for each of the equivalent signal point sequences and a selection element configured to select a preferred signal point sequence based on a comparison of the measurable quantity for a number of equivalent signal point sequences.